The present invention relates to an antenna switching circuit used in a diversity receiver in a mobile radio set.
The strength of the electric field received by an mobile radio set fluctuates over a wide range due to fading. In order to avoid the effects of fading, a diversity receiver, shown in FIG. 3 is used. Separately provided antennas 21 and 22 are provided and used to concurrently receive the signal from the base station, and an antenna switching circuit (DIV/SW) 25 controlled by a controller (CPU) 27 selects one of the signals having the higher level, and supplies the selected signal to a receiver 26. The antennas 21 and 22 are configured to exhibit different sensitivities for signals received via different propagation paths. For instance, they are disposed at different heights, or optimized for different frequencies. One of the antennas 21 and 22 are used also for transmission. In the illustrated example, the antenna 21 is used for reception only and is called a receive antenna (R-ANT), while the antenna 22 is used for reception and transmission and is called a transmit/receive antenna (T-R-ANT).
A known the antenna switching circuit (DIV/SW) 25, is comprised of PIN (p-type/intrinsic/n-type) diodes FIG. 4 is a circuit diagram showing the antenna switching circuit employing PIN diodes D31 to D34.
Referring to FIG. 4, a receive signal received by a receive antenna (R-ANT) shown in FIG. 3 is applied to an input terminal 31, and a receive signal received by a transmit/receive antenna (T-R-ANT) is applied to an input terminal 32. The receive signal selected by the switching circuit is sent out via an output terminal 33 to a receiver 26 shown in FIG. 3. Control signals are applied to terminals 34, 35. When a voltage is applied in a forward direction to a PIN diode, a current flows and the resistance is decreased. When a voltage is applied in a reverse direction to a PIN diode, a current does not flow and tile resistance is increased. The impedance of the capacitors C31 to C38 are set to be sufficiently small at the operating frequency.
When a control signal of a high level, e.g., 5 V, is applied to the terminal 34 and a control signal of a low level, e.g., 0 V, is applied to the terminal 35, a forward voltage is applied to the PIN diodes D32 and D33 so that a current flows through them and their resistances are low, while a reverse voltage is applied to the PIN diodes D31 and D34 so that no current flows through them and their resistances are high. Accordingly, the path between nodes N0 and N2 is conductive, while the path between the nodes N0 and N1 is nonconductive. The receive signal applied to the input terminal 32 is therefore output via the output terminal 33. Conversely, when a control signal of 5 V is applied to the terminal 35 and a control signal of 0 V is applied to the terminal 34, a forward voltage is applied to the PIN diodes D31 and D34 so that a current flows through them and their resistances are low, while a reverse voltage is applied to the PIN diodes D32 and D33 so that no current flows through them and their resistances are high. Accordingly, the path between nodes N0 and N1 is conductive, while the path between the nodes N0 and N2 is nonconductive. The receive signal applied to the input terminal 31 is therefore output via the output terminal 33.
In order to increase the reception sensitivity of the diversity receiver, it is necessary to minimize the insertion loss of the antenna switching circuit (DIV/SW) 25 provided in front of the receiver 26.
In order to minimize the insertion loss of the antenna switching circuit of the configuration shown in FIG. 4, it is necessary to reduce the forward resistances of the PIN diodes D31 and D33, and as a result it is necessary to cause a current of a large value, such as 5 mA, to flow through the PIN diodes D31 and D33. This is a disadvantage in a mobile radio set in which the power supply is formed of a small-sized battery.
Moreover, in the above-described antenna switching circuit, the insertion loss cannot be eliminated, and when it is provided in front of the receiver 26 shown in FIG. 3, reduction in the reception sensitivity cannot be avoided.